This invention relates to the electrification of a fence e.g. for security or for the control of livestock and more particularly is concerned with enhancing the efficiency and safety of a fence energiser.
The electric fencing industry is highly competitive. Constant efforts are being expended by various parties to increase the effectiveness of energisers and to reduce the number of energisers required for a given application. The maintenance of an electric fence is also important and ongoing work is being done to reduce the amount of human intervention required to maintain an electric fence as an effective barrier.
A problem which is inherent in the control of an electric fence is that the output voltage of an energiser, in the order of 8000 volts, is difficult to switch in a cost-effective way without human intervention.
Many energisers have been proposed which generate unipolar outputs, primarily for historic reasons and because of circuit requirements.
WO03026362 proposes a bipolar output energiser for overcoming poor earthing conditions. Bipolar output energisers have also been described in international applications PCT/ZA2005/000179, PCT/ZA2005/000180 and PCT/ZA2006/000089.
It is known to design an energiser with more than one output terminal to enable different output energy levels to be offered. Mechanically operated, high voltage switches are often used in agricultural applications to enable a user to select a strand or fence to be energised. The use of a mechanically operated switch is labour intensive, inconvenient and undesirable.
An energiser which uses an electrically actuated switching arrangement is shown for example in AU 198547506. It appears though that a single output from an energiser is switched to different wires in a fence.
Various techniques have been suggested to increase the effective output energy of an energiser while still remaining within prescribed safety limits in order to reduce the number of energisers required for a given application. These include feedback schemes to increase the output energy of an energiser according to the loading of a fence to which the energiser is connected.
Prior art documents representative of the aforegoing include the following: AU198826906, NZ270570, WO88/10059, U.S. Pat. No. 6,020,658, NZ509061, WO00/35253, GB2403856 and PCT/2006/000089.
In many instances energiser safety has been increased significantly. It is observed however that to the applicant's knowledge it has not been possible to operate an energiser to compensate for unnecessary losses on a fence.
A delayed response energiser system has been proposed in GB2403856. However it falls short of a real-time measurement and real-time response electric fencing application.
An increase in energiser output energy raises concerns about the resulting safety to humans and animals. In some proposed real-time energy control mechanisms known to the applicant the cause of the increased fence losses has not been identified and the fence losses have not been minimised. For example it may happen that at a single point on a fence significant loading occurs and the whole fence is compromised. The applicant is unaware of a technique which enables the selective measurement of energy losses at different points along a fence to be effected and for such losses to be reduced or eliminated, without mechanical intervention.
A further complicating factor is that the losses incurred along a fence may not be linear due to the fact that very little loading takes place until ionisation of the air adjacent the fence occurs. Control algorithms known to the applicant attempt to maintain the fence voltage at a given set point. However, if it is possible to operate at a lower set point and simultaneously provide an adequate fence voltage level, losses in wasted energy can be substantially reduced. In this respect reference is made to FIG. 1 of the attached drawings which is a graph of fence energy losses versus fence voltage. It is seen that the losses increase exponentially with an increase in fence voltage. With this type of information a control algorithm can be used to choose a suitable operating point of, say, 5000 volts in order to ensure a minimum fence voltage of 3000 volts e.g. in an agricultural application.
Traditionally, as is shown in FIG. 2, an energiser 200 in an electric fence installation for a security application has a loop 202 for monitoring the voltage at an end 204 of a fence. The monitoring of the voltage at the end of the fence may also be used in agricultural applications. The return voltage is monitored and a control algorithm is used to try and maintain the voltage on the fence. A waveshape 206 of such an energiser is shown in FIG. 3. A disadvantage of such a system is that, should arcing occur at some point along the fence, at a certain voltage, any increase in voltage at the output terminals of the energiser will not increase the voltage at any point further along the fence from the arcing point.
FIG. 4 has a graph A of fence voltage against distance, with normal loading. A curve B shows the effect of an arc on a fence at a point X and of increasing the output voltage. FIG. 5 indicates the effect on the fence pulse 206. The total area (C+D) under the curve is proportional to the energy contained in the pulse. If there is no arcing the full pulse energy is available. The pulse energy is reduced to the portion C if there is arcing. It is apparent that by increasing the output voltage of the energiser very little is achieved in terms of available energy on the fence after the arcing point. The danger of a simple, closed-loop, voltage control when arcing is present on the fence is that the energy level at the output terminal may reach a potentially lethal value.
One method of overcoming an electric security fence is to connect the energiser output wires and return wires to one another. A portion of the existing fence is then removed. Clearly this activity affects fence energy consumption but, to the applicant's knowledge, this factor has not been used for alarm activation.
Another method of overcoming an electric security fence is to place or throw one or more lengths of bare wire, each in the shape of a hook, onto the fence. An electric security fence typically has alternate live and earth strands. Each wire then electrically short-circuits the strands of the fence making it possible to climb through the fence because the potential is effectively zero in close proximity to the location of the short circuit.
The invention aims to provide a selective fence energiser system which, at least partly, addresses the aforementioned problems.